Two-component polyurethane forming coating compositions are widely used because of the many advantageous properties they exhibit. These coating compositions generally comprise a liquid binder component and a liquid hardener/crosslinker component. The liquid binder component may comprise an isocyanate-reactive component, such as a polyol, and the liquid crosslinker component may comprise a polyisocyanate. The addition reaction of the polyisocyanate with the isocyanate-reactive component, which can occur at ambient conditions, produces crosslinked polyurethane networks that form coating films. Polyurethane coatings are used in a wide variety, one of which is often referred to as “In-Mold” coating.
In an In-Mold coating application, a coating film is molded over the surface of a molded plastic substrate. In an In-Mold coating method that utilizes a multi-cavity metal mold, the molded plastic part is formed in one cavity of the mold and the molded plastic part is introduced into a second cavity of the mold in which the coating film is injected. Such a process can have advantages over a single cavity in-mold coating process. For example, cycle time is shorter since it is not composed of the sum of the times of the individual process steps and process parameters can be chosen independently for each cavity.
A historical problem with In-Mold coating processes including those using two-component polyurethane forming compositions, such as process that utilize a multi-cavity metal mold, is that of inadequate “demolding” after the coating has been applied, cured, and the mold opened. “Demolding” refers to the ability of a coating molded plastic substrate to release easily from the metal mold when it is opened. Poor demolding means that the coated plastic substrate adheres to the surface of the metal mold such that a significant external force is needed to remove the coated substrate from the mold. This external force often causes product quality and productivity issues, for example. Of course, other important coating properties, such as adhesion of the coating to the molded plastic substrate, and physical properties, such as scratch resistance, stain resistance, and chemical resistance, are also important.
As a result, it would be desirable to provide a process for in-mold coating utilizing a multi-cavity metal mold that exhibits excellent demolding properties, i.e., the coating plastic molded substrate releases, i.e., demolds, from the coating mold cavity via gravity alone or with a minimal external force (such as suction) when the mold is opened, so that the coated molded substrate is not deformed or otherwise damaged by its removal from the mold while providing a coated substrate that has an high quality finish exhibiting good physical properties, such as scratch resistance, stain resistance, and chemical resistance, while also adhering well to the molded plastic substrate.
The present invention was made in view of the foregoing.